An optical multiplying modulator performing an optical modulation by a multiplying modulation has already been known (see Japanese Patent No. 3404528, Japanese Patent No. 3343241, Japanese Patent Application Laid-Open Publication No. 2002-148572, and Japanese Patent No. 3496053). FIG. 8 is a block diagram (FIG. 1 of Japanese Patent No. 3404528) showing a basic principle of a conventional optical frequency modulator by a multiplying modulation. This optical multiplying modulator is a frequency converter by the multiplying modulation provided with a structure for modulating a light of a predetermined frequency to obtain a secondary sideband wave group, a structure for modulating an nth order sideband wave group to obtain an (n+1)th order sideband wave group, and a structure for selecting a specific singe sideband wave from among a plurality of sideband wave groups by using a narrowband filter (claim 1 of Japanese Patent No. 3404528).
As shown in FIG. 8, a more specific basic arrangement of an optical multiplying modulator (101) is provided with an optical modulator (102), a first filter (103) for controlling a light inputted to and outputted from the optical modulator (102), a second filter (104) for controlling a light inputted to and outputted from the optical modulator (102), and a modulating signal source (105) for inputting a modulating signal to the optical modulator (102). The first filter (103) is a narrowband filter that has a characteristic of reflecting a light marginally deviated from a certain frequency f0 and transmitting lights having other frequencies. Also, the optical modulator (102) is an optical intensity modulator (or an optical phase modulator) capable of modulating an input light in whichever direction of a leftward direction and a rightward direction by a characteristic of a modulating frequency fm. The second filter (104) is a filter (bandlimiting filter) having a characteristic of transmitting a light having a specific frequency (e.g. the third sideband waves) and reflecting other lights.
The basic operation of the optical multiplying modulator is as follows: FIG. 9 is a schematic diagram showing the basic operation of the optical multiplying modulator. As shown in FIG. 9(A), an input light has a single frequency f0 [Hz]. The light of the frequency f0 having been inputted through the first filter (103) is modulated by the optical modulator (102) and generates sideband waves as shown in FIG. 9(B). (It is to be noted that for the sake of simplicity, it is supposed that only the carrier wave f0 and the primary sideband wave group (f0−fm and f0+fm) are generated.) The carrier wave and the primary sideband wave group are reflected by the second filter (104) and pass through the optical modulator (102) again. Upon passing through the optical modulator (102), these lights are modulated and outputted as lights having spectrums shown in FIG. 9(C). Among these lights, the carrier wave f0 passes through the first filter (103). Namely, spectrums of lights reflected by the first filter (103) are as shown in FIG. 9(D). These lights reflected by the first filter (103) pass though the optical modulator (102) again. Then, these lights are modulated by the optical modulator (102) and become lights having spectrums shown in FIG. 9(E). Namely, light groups shown in FIG. 9(E) has the primary sideband wave group (f0±fm) and the third-order sideband wave group (f0+3fm). Among these, the primary sideband wave group shown in FIG. 9(F) is reflected by the second filter (104). It is to be noted that these light groups are lights having similar frequencies with those of the light groups shown in FIG. 9(B). On the other hand, the third-order sideband waves pass through the second filter (104). Thus, the third-order sideband waves are outputted from the optical multiplying modulator as output lights. By controlling a transmitting region of the second filter (104), the frequencies of the lights obtained as the output lights can be controlled. Namely, by thus using the optical multiplying modulator, a high-order sideband wave group can be obtained. It is to be noted that for the optical modulator (102), modulators such as an intensity modulator and a phase modulator can be mentioned.
An FSK (frequency shift keying) technology has already been known. For example, an invention related to an optical transmission system capable of performing a good demodulation without using electric components for broadband and high frequency is disclosed by the Japanese Application Laid-Open Publication No. 11-331089. As an angle modulating signal, an FSK modulating signal that is a digital signal whose frequency is modulated is used.
Also, a millimeter wave FSK transmitting and receiving system where one parent station and “N” unites of child stations are wirelessly communicating in a millimeter waveband that requires a high-quality communication in an in-house communication that generates multiple reflection is disclosed by the Japanese Application Laid-Open Publication No. 10-150380.
Also, a circuit configuration technology for configuring a receiver receiving an FSK signal by a direct conversion receiving method is disclosed by the Japanese Application Laid-Open Publication No. 09-224059.